Non-integer scaling of raster images with image quality enhancement

Abstract

A method for scaling an input bitmap by a non-integer factor with improved image quality comprises the steps of: selecting a pel in the input bitmap; selecting a neighborhood surrounding the selected pel; searching a set of image feature patterns to find an image feature pattern that matches the selected neighborhood; and writing a blocks of bits to an intermediate bitmap based on the match. The appropriate block of bits is obtained by subsampling an image enhancing tile which corresponds to the matched image feature pattern. The process of matching neighborhoods and writing a subsampled image enhancing tile (i.e., block of bits) to the intermediate bitmap is repeated for each pel in the input bitmap. When the image enhancing tiles are subsampled to the proper size and shape and placed in the proper locations in the intermediate bitmap, the jagged “stair-step” look of curved and diagonal image features in the input image is smoothed, resulting in improved quality. The intermediate bitmap is then scaled down to a selected output resolution and written into an output bitmap with improved image quality and resolution greater than that of the input bitmap. This method is particularly useful for scaling raster font characters from a given input resolution to the resolution required by an output device when the resolution of the output device is a non-integer multiple of the input resolution.